1. Field of the Invention
This invention relates to a printing head assembly for use with an ink-jet type printing machine.
2. Relevant Art
An ink jet type printing head assembly using vapor pressure to assist in the application of ink is disclosed for example by Allen et al., "Thermodynamics and Hydrodynamics of Thermal Ink Jets", Hewlett-Packard Journal, May 1985, pp. 21-26.
FIG. 3 illustrates a prior art ink jet type of printing head assembly which uses air pressure to assist in the application of ink. In FIG. 3, reference numeral 1 denotes a front panel and reference numeral 2 denotes a rear panel of the assembly. A number of front nozzles 4, each having an inner diameter of approximately 50 .mu.m, are formed on the front panel 1, while a corresponding number of rear nozzles 5 are formed on the rear panel 2 in an aligned relationship. One common electrode 6 is provided on the front surface of the front panel so that it covers the front nozzles 4, while a plurality of separate electrodes 7 are provided on the rear surface of the rear panel 2 so that they each cover a respective one of the rear nozzles 5. An ink chamber 8 is provided behind the rear panel 2. A spacing between the rear panel 2 and the front panel 1 defines an air chamber 9. The ink chamber 8 receives ink from an ink reservoir 10, and the latter reservoir 10 is pressurized by an air pump 11. The air pump 11 also pressurizes the air chamber 9.
The printing head assembly described above functions in the following manner.
When a voltage is applied between one or more of the separate electrodes 7 and the common electrode 6, an electro-static type of attractive force is generated to pull an ink droplet out from each activated rear nozzle 5 into the air chamber 9. As a result of air supplied by the air pump 11 to the air chamber 9, air jet streams are formed in the air chamber 9 and these blow out of front nozzles 4. The ink droplets which have been pulled out of the ink chamber 8 into air chamber 9 by the electro-static attractive force spout out from the front nozzles 4 in the form of many tiny droplets and these tint a paper surface (not shown) placed near the front nozzles 4.